1. Field of the Invention
The present invention relates to a mobile communication system. More particularly, the present invention relates to an apparatus and method for Forward link Outer Loop Rate Control (FOLRC) a using Hybrid Automatic Repeat reQuest (HARQ) scheme.
2. Description of the Related Art
Mobile communication systems employ a Hybrid Automatic Repeat reQuest (HARQ) scheme (i.e., a physical layer packet retransmission algorithm) in order to increase forward link throughput. In a system employing the HARQ scheme, a Mobile Station (MS) transmits Acknowledgement (Ack) or Negative-acknowledgement (Nack) in response to a packet transmitted from a Base Station (BS) according to success or failure in receiving the packet in a forward link physical layer. Upon receiving the Ack, the BS subsequently transmits a next packet to the MS. Upon receiving the Nack, the BS retransmits a previously transmitted packet to the MS. In this case, instead of discarding the previously received packet, the MS stores the previously received packet in a buffer. When the same packet is retransmitted at a later time, the MS attempts to perform demodulation by combining the retransmitted packet with the previously transmitted packet. Therefore, the HARQ scheme has an advantage in that link throughput can be improved by increasing a probability that a packet is successfully received.
The BS performs Forward link Outer Loop Rate Control (FOLRC) for the purpose of satisfying a Carrier to Interference and Noise Ratio (CINR) value which is requested in an adaptive manner to a changing channel condition. An existing system has a large number of link tables optimized to a channel model (i.e., a movement speed and a surrounding environment). The BS determines a transmission method according to a channel condition of the MS by using a link table fixed to a current channel model. The link table is defined as a table in which Modulation and Coding Scheme (MCS) levels are pre-defined for all CINR values. The BS may evaluate a CINR value by using information (i.e., a Channel Quality Indicator (CQI)) on a data transfer size that can be received from the MS, and allocate an MCS level corresponding to the evaluated CINR value to the MS. In this case, a CINR range occupied by each MCS level in the link table indicates a range in which the MS can successfully receive data with a Packet Error Rate (PER) of less than 1% when the BS transmits to the MS the data coded and modulated according to the assigned MCS level.
The link table is optimized to a worst-case channel condition in order to avoid errors. Since the CQI does not have overall channel information, it is difficult for the link table to correctly indicate the channel condition. Accordingly, when the link table fixed as mentioned above is used, channel adaptation significantly deteriorates. Therefore, although the BS is able to allocate a higher order MCS level to the MS, the BS allocates a lower order MCS level to the MS by depending entirely upon the currently operating link table, i.e., a data transfer rate is assigned to have a relatively smaller system capacity than a currently available system capacity. Therefore, when the aforementioned fixed link table is used, in order for the BS to adaptively allocate an MCS level according to the channel condition of the MS, an additional channel estimation algorithm that can predict the channel condition is required, and switching must be performed on the link table by using this algorithm.